Recent advancements in display technology are beginning to allow for an extended range of color, luminance and contrast to be displayed.
Technologies allowing for extensions in luminance or brightness range of image content are known as high dynamic range imaging, often shortened to HDR. HDR technologies focus on capturing, processing and displaying content of a wider dynamic range.
Although a number of HDR display devices have appeared, and image cameras capable of capturing images with an increased dynamic range are being developed, there is still very limited HDR content available. While recent developments promise native capture of HDR content in the near future, they do not address existing content.
To prepare conventional (hereon referred to as SDR for Standard dynamic range) content for HDR display devices, reverse or inverse tone mapping operators (iTMO) can be employed. Such algorithms process the luminance information of colors in the image content with the aim of recovering or recreating the appearance of the original scene.
Different kinds of inverse tone mapping algorithms exist, as for instance local tone mapping algorithms and global tone mapping algorithms. For example, in the field of local tone mapping algorithms, the patent application WO2015/096955 discloses a method comprising, for each pixel p of the image, the steps of obtaining a pixel expansion exponent value E(p) and then inverse tone mapping the luminance Y(p) according to the equationYexp(p)=Y(p)E(p)  (1)
wherein Yexp(p) is the expanded luminance value of the pixel p.
The set of values E(p) for all pixels of an image form the expansion exponent map for this image. The expansion exponent maps can be generated by different methods, for example by low-pass filtering.
Dedicated tools have been developed to implement inverse tone mapping methods to SDR images. These tools can be used either as a plugin within a post-production tool to help colorists for manual grading, or as hardware implementation within a SoC (System On Chip) for a set-top box or TV set. In the first case, the images are manually processed by colorists. The process gives good results (the artistic intent of the film makers can be preserved) but cannot be performed on the fly. In the latter case, predetermined expansion parameters are applied to the video sequences without any adaptation to the video content and without the intervention of colorists. The inverse tone mapping can thus be performed on the fly. But the results are not so good as those issued from a manual grading of colorists.
There is a need to set a global tone mapping algorithm that can adapt automatically to the content to tone-map.